The present invention relates to a surface-mounted antenna which is suitably incorporated in a portable telephone, a portable wireless device or the like, and is small in size and may be directly mounted on a surface of a printed circuit board. More particularly, the invention relates to a surface-mounted antenna in which a feeding electrode is highly efficiently coupled to a radiation electrode by improving the electrical coupling of a feeding electrode with a radiation electrode, and a portable wireless device using the same.
A PIFA (planar inverted-F antenna), called an inverted-F antenna, and a forefront capacitive feeding inverted-L antenna are frequently used for the surface-mounted antenna of this type which may be reduced in size. The inverted-F antenna has the following structure as roughly sketched in FIG. 4. A conductive film is formed ranging from one main surface of a dielectric substrate 21 to a side surface thereof. A radiation electrode 22 is formed such that one end thereof is open, and the other end thereof, located closer to its side surface, is connected to a ground electrode 23 provided on the rear side of the dielectric substrate. A feeding pin 24 is connected to a feeding part 22a, which is located closer to its connection terminal connecting to a ground electrode 23, through a through-hole passing through the dielectric substrate 21 and the ground electrode 23.
The inverted-L antenna, as shown in FIG. 5, for example, is provided on a surface of a dielectric substrate 21 such that its radiation electrode 22 is confronted with a feeding electrode 24, and is capacitively coupled with the same. A ground electrode 23 is provided on the reverse side of the dielectric substrate 21. In the structure of the inverted-L antenna, the radiation electrode 22 is open at one end, and capacitively coupled with the feeding electrode 24, and connected at the other end to the ground electrode 23.
In each of those antennas, the radiation electrode is open at one end, while the other end is grounded, and has an electrical length of about λ/4 (λ=wavelength of the operation frequency). The radiation electrode is excited and operated in a resonance mode. The operation frequency (resonance frequency) of the antenna is determined mainly by an electrical length of the radiation electrode. Advantageously, the operation frequency is adjusted by adjusting the length of the radiation electrode substantially independently. An additional advantage is that in both types of antennas, the impedance matching for feeding to the radiation electrode is performed independently of the operation frequency.
In the inverted-F antenna, the radiation electrode is open at one end (maximum voltage) and grounded at the other end (zero voltage), and the radiation electrode is connected to the feeding pin at a point at which the impedance of the feeding pin is made coincident with the impedance of the radiation electrode, which is located closer to the ground terminal. Accordingly, in a case where the impedance at the feeding point to which the feeding pin is connected becomes different from that of the feeding pin as a result of the operation frequency adjustment of the radiation electrode, the necessity of moving the connection point occurs to change a connecting position of the feeder line. Accordingly, its continuous adjustment is difficult.
Also in the forefront capacitive coupling inverted-L antenna, a coupling gap is provided between the open end of the radiation electrode and the feeding electrode. Those are capacitor coupled with each other through the gap. Advantageously, in the inverted-L antenna, the impedance matching is carried out independently of the operation frequency adjustment, by adjusting the gap size. However, disadvantageously, in the inverted-L antenna, when the open end of the radiation element is moved in order to change the operation frequency of the radiation electrode, the gap size resultantly changes. Consequently, it is impossible to perform the impedance matching perfectly independently of the operation frequency adjustment.
A quantity of capacitance coupling theoretically depends on a dielectric constant and dielectric effects. Therefore, the coupling loss resulting from the dielectric loss inevitably occurs. This causes the antenna loss. Further, the capacitive coupling part is theoretically located at a maximum point of electric field. Accordingly, an electric field distributed around the capacitive coupling part interacts with a dielectric material present around the capacitive coupling part, and a coupling quantity tends to vary. As a result, the matching characteristic is apt to vary.
Further, the feeding electrode is open at the tip end (forefront) thereof. It exhibits a high impedance characteristic over a broad frequency range from frequencies lower than the operation frequency band to DC. Accordingly, the antenna is sensitive to incoming noise and static electricity, and is apt to impart load to the device installed with the antenna.
Additionally, theoretically, the coupling capacitance is sensitive to the coupling gap dimension. Accordingly, the matching characteristic is sensitive to a change of the gap dimension, and the matching characteristics of the products tend to vary at the stage of their manufacturing.